The present invention relates to semiconductor devices and their manufacture, and more particularly to the incorporation of charge carrier lifetime controlling impurities into semiconductor power devices.
In many types of semiconductor devices, it is, desirable to provide localised areas with short carrier lifetimes relative to the remainder of the device. This may be achieved by the introduction of recombination centres. Two methods typically used are the diffusion of lifetime controlling impurities such as gold or platinum and high energy particle irradiation. However, it is difficult to control the recombination centre distribution obtained by diffusion of gold or platinum and it generally results in less than optimal xe2x80x9cbath tubxe2x80x9d concentration profiles with few recombination centres deep in the structure. The recombination centres produced using irradiation are mainly restricted to surface regions of the device. Also, the lattice damage caused by the high energy irradiation needed to create deep recombination centres is susceptible to annealing by subsequent heating even at low temperatures, which may reduce the long term stability of the device. A localised short carrier lifetime region well below the surface is therefore difficult to obtain using either method. Furthermore, neither approach is suited to lateral masking because of the inevitable lateral spread during diffusion, and because any radiation that is energetic enough to penetrate deep into the silicon is hard to mask. Thus lateral confinement of the recombination centres is problematic using either process.
An example of a device in which a localised distribution of recombination centres would be particularly beneficial is the P-i-N rectifier. Such devices are discussed, for example, in xe2x80x9cPower Semiconductor Devicesxe2x80x9d by B. Jayant Baliga (1995), (hereinafter referred to as xe2x80x9cBaligaxe2x80x9d), at pages 153 to 182, the contents of which are hereby incorporated herein as reference material. Lifetime control is described on pages 175 to 177 thereof, where it is noted that a narrow distribution of recombination centres gives an improved trade-off between forward drop and reverse recovery speed. However, it is acknowledged that it is difficult to achieve such a distribution using the methods discussed above. There has thus been a long-standing and widely recognised need for a viable solution to this problem.
The article xe2x80x9cInfluence of Carbon Concentration on Gold Diffusion in Siliconxe2x80x9d by M. J. Hill (one of the present inventors) and P. M. Van Iseghem, published in the Proceedings of the Third International Symposium on Silicon Materials Science and Technology of the Electrochemical Society, 1977, Vol. 77-2, pages 715 to 725, investigated the homogeneity of gold diffusion in thick silicon slices for power devices. The distribution of diffused gold had been found to depend on many factors, including the source of the silicon. In the article, it is suggested that variations in carbon concentration across the silicon influence the gold distribution and that carbon levels varied within the same grade of FZ silicon and also between silicon from different manufacturers.
It is an object of the present invention to provide improved control over the incorporation of lifetime controlling impurities into semiconductor devices.
The present invention provides a semiconductor device having a semiconductor body formed substantially of silicon, the body containing an active device area in which charge carriers flow during operation of the device, and the active device area including a region having a predetermined concentration of carbon, wherein a lifetime controlling impurity is provided in the body which is substantially located in the carbon region. The inventors unexpectedly found that by introducing a predetermined concentration of carbon into a localised region of the active area of a device where a reduced carrier lifetime is desired, it is possible to closely control the distribution of a lifetime controlling impurity there, substantially restricting it to a predefined location. This enables much greater control over carrier lifetime in different regions of device structures and therefore their operating characteristics and uniformity.
The carbon atoms occupy lattice sites in the silicon of the semiconductor body. Although these atoms are electrically neutral, the inventors believe that when larger impurity atoms such as gold or platinum are introduced it is energetically favourable for these impurities to occupy lattice sites adjacent to the smaller carbon atoms to reduce local lattice strain. The impurity becomes associated with the predetermined concentration of carbon. It is thought by the inventors that the association between the impurity ions (M+) and the carbon atoms (C) on lattice sites produces C-M+ complexes with significant capture cross-sections.
Typically, the concentration profile of the impurity in the carbon region substantially corresponds to that of the carbon, the carbon profile being predetermined and controlled. Thus, the carbon may be employed to impose a specific profile on the impurity, according to the requirements of a particular device configuration. The extent to which the impurity is taken up by the carbon region may be controlled by careful control of the temperature the device is exposed to during its manufacture. Temperatures lower than normally used for diffusion of a lifetime controlling impurity may be sufficient to achieve the desired result owing to the association between the carbon atoms and the impurity. The reduced temperatures may provide a more confined overall impurity distribution.
The carbon may be substantially laterally and/or vertically confined within the semiconductor body.
The invention further provides a method of manufacturing a semiconductor device having a semiconductor body formed substantially of silicon, the body containing an active device area in which charge carriers flow during operation of the device, the method comprising the steps of providing a region in the active device area having a predetermined concentration of carbon, and heating the body such that a lifetime controlling impurity within the body becomes substantially located in the carbon region.
The carbon region may retain one or more lifetime controlling impurities. The xe2x80x9cbackgroundxe2x80x9d level of the one or more impurities generally present in a high temperature furnace (diffusing from the internal walls, for example) may be sufficient that the concentration thereof caused by the carbon provides the desired degree of localised lifetime control. Otherwise, the method may include the step of actively introducing the lifetime controlling impurity into the body prior to the heating step.
The heating step may be carried out specifically to associate the impurity with the carbon region, or it may form part of another process, later in the fabrication of the device.
Carbon atoms may be provided in the semiconductor body of a device in several ways. For example, they may be implanted and/or diffused in the body, before, during and/or after other high temperature processes. A mask may be used to laterally confine the implantation or to restrict the surface area exposed during diffusion of impurities into the body. Preferably, it is introduced during growth of an epitaxial layer of silicon. This approach is relatively low cost, and the distribution (that is, the concentration) of the added carbon vertically, and laterally where appropriate, can be accurately controlled and may be restricted to a well defined, discrete region or regions. In a further technique, the carbon is incorporated uniformly during bulk growth of a silicon substrate. It may then provide greater control over the subsequent diffusion, and thus distribution, of a lifetime controlling impurity.
Thus, using the techniques described herein, it is possible to introduce carbon into the semiconductor body of a device in a predetermined and controlled manner, to produce a desired carbon concentration profile in a localised region in the active area of the device. It may be preferable to add the impurity relatively late in the manufacturing sequence to minimise any alteration of its distribution by subsequent processes, although pinning of the impurity by the carbon will tend to reduce its susceptibility to movement by later heating.